1. Field of the Invention
The invention relates to a method for making rare earth element, iron and boron sintered permanent magnets from dross or alloy scrap containing a rare earth element, iron and boron, and to an alloy particle mixture for use therein.
2. Description of the Prior Art
During the production of permanent magnet alloys of a rare earth element-iron-boron in a vacuum induction furnace approximately 5 to 10% of the furnace charge material is formed into dross during this melting operation. This dross typically contains very high oxygen and nitrogen contents, on the order of 0.5 to 4.5 weight % and 0.1 to 3.0 weight %, respectively. Upon the conclusion of the vacuum induction melting operation, the dross is formed on the surface of the melted alloy and thus may be readily removed from the furnace. The dross contains Nd.sub.2 Fe.sub.14 B matrix phase, Nd oxide and Nd nitride, and an alpha iron phase. Because of the very high oxygen and nitrogen contents, it is difficult to densify dross to levels sufficient for the production of permanent magnet articles by conventional sintering practices. Consequently, dross is typically disposed of as a waste material.
In the manufacture of rare earth element-iron boron permanent magnets by conventional powder metallurgy techniques, such as sintering, there are some magnets that do not meet the manufacturing specifications, such as magnetic properties, dimension, physical integrity, corrosion resistance and the like. These magnets are thus rejected as scrap or revert material. It is difficult to reprocess scrap directly into permanent magnet articles by conventional powder metallurgy techniques, such as sintering. Specifically, if the scrap is crushed to form powder and the powder is pressed and sintered in a conventional manner, it is difficult to achieve the required high densities for permanent magnet articles because of the relatively high oxygen content, e.g. 0.3 to 1.5% of the scrap.
Attempts have been made to recover the rare earth element-iron-boron alloy content from dross and scrap materials by various processes, including remelting the dross or scrap with reducing agents such as calcium (Ca) or calcium hydride (CaH.sub.2), reduction and diffusion or thermite reduction. With remelting practices, the yields are extremely poor and generally do not warrant the remelting cost involved. In addition, the remelted alloy is generally contaminated from the reducing agents and refractory materials to render it unsatisfactory for use in permanent magnet production.